Electrolytic sensors for detecting and measuring the pH of a liquid system (a measurement of the hydrogen-ion activity) are well known. Generally such pH sensors include a glass membrane electrode and a reference electrode. The glass electrodes tend to be quite fragile, and are therefore not generally suitable for applications where the electrodes are subjected to a considerable amount of movement, jostling or shock, or high temperatures or pressures.
Junction-type metal/metal oxide solid state pH electrodes have been proposed for sensing the pH of solutions and other fluids. These electrodes have the sought after advantages of stability in aqueous solutions over a wide range of temperatures and pressures, low impedance and fast response to pH changes. Fog et al., "Electronic Semiconducting Oxides as pH Sensors", Sensors and Actuators, 5 (1984) 137-146, discuss the limitations of such pH sensors. Oxidizing and reducing agents, such as ferricyanide, ferrocyanide and hydrogen peroxide were found to interfere with pH measurement. In addition, pH sensors which utilize the junction-type electrodes discussed therein, retain the limitations of the glass electrodes discussed above, when coupled with a conventional reference electrode.
Various improvements have been made on the junction-type electrode to make it more rugged and compact.
U.S. Pat. No. 3,905,889 discloses a pH sensor in which the reference and indicator electrodes are surrounded by an electrolyte and encased in a hydrogen ion and carbon dioxide permeable diffusion barrier, such as poly(siloxane)-poly(bisphenol-A) polycarbonate block copolymer. The effective pH range for this probe is very limited, from 5.6 to 7.1.
U.S. Pat. No. 4,536,274 discloses a transcutaneous blood carbon dioxide sensor which utilizes a junction-type electrode of palladium/palladium oxide and a silver/silver halide electrode applied to an electrically nonconductive substrate, partially coated with an insulated dielectric and the remainder thereof optionally coated with any of a number of polymeric membrane materials, including perfluorocarbon copolymers. This pH sensor is limited to measuring a narrow pH range of from 6.49 to 8.50, and is characterized by slow responsiveness and poor reproducibility.
Though these electrodes may be more rugged than the glass membrane electrodes, inherent mechanical sealing problems exist with these electrodes, including glass membrane electrodes, particularly when utilized for on-line pH monitoring in high pressure and/or high temperature processes. For example, compression seals in such applications generally utilize sealing polymers which flow when heated causing the seal to break down and leak upon temperature cycling. To complicate matters, those sealing polymers which do not flow upon heating generally take-up water, i.e., hydrate (0.06% w or more water), leading to a low resistance leakage path between electrodes. This leakage path essentially short circuits the electrodes resulting in an erroneous pH reading.
Thus, there exists a need to make a pH sensor and electrodes therefor which are rugged, compact and seal-less.